Really dumb question - but did you check the faces of the contact plates while you had it apart? Were there any grooves, however slight?
Typically, when I rebuild an NSS, I'll take the contacts in a pair of pliers and run them over some 400 grit emery (in a pinch, a fingernail emery board also works neatly) to "refresh" the contact faces - which not only removes crud, but also helps to smooth them out again. After I clean them up, I'll run a thumbnail down the contact face and check for grooves - if I find any, I'll repeat the emery until they're gone.
Also, I'll only give a VERY LIGHT coating of Ox-Gard or Dielectric grease - nothing that is not meant for electrical use (like white lithium grease.) Ox-Gard probably works the best, and you don't need to pack the switch with the stuff - just a light coating on the contact strips and the wiper arm will do. Then, a VERY LIGHT bead of RTV black on the mating surfaces of the case halves to seal it against moisture intrusion - like Brylcreem, "A little dab'll do ya!" Don't get stupid with the stuff...
I've also been known to break down a fingernail board and use it to clean up the contact strips in the switch shell - not much here, but remove any varnish and crap buildup. Clean with contact cleaner or some variety of alcohol solvent (like denatured alcohol or methanol) to remove any leftover grit. Don't use booze - not only is there stuff in booze that will inhibit conduction, but it's a waste of perfectly good spirits...
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